In engineering, a foundation is the element of a structure
which connects it to the ground or more rarely, water (as with floating structures),
transferring loads from the structure to the ground. Foundations are generally
considered either shallow or deep. Foundation engineering is the application of
soil mechanics and rock mechanics (geotechnical engineering) in the design of
foundation elements of structures.
All structures are provided with foundation at the base to fulfil
the following objectives and purposes:
To distribute the load of the structure over a large bearing
area.
To load the bearing surface at uniform rate so as to avoid
unequal settlement.
To prevent the lateral movement of the supporting material.
To increase the stability of the structure as a whole.
Foundation are classified on the basis of load transmission
to the ground into two sub-categories i.e. shallow foundation and deep
foundation.
Shallow Foundation
Shallow foundation are those foundations in which the depth
at which the foundation is placed is less than the width of the foundation (D
< B). Shallow foundations are generally termed as spread footing as they
transmit the load of the super structure laterally into the ground.
Classification of Shallow Foundation:
On the basis of design, the shallow foundation are
classified as:
1.
Wall Footing
2.
Isolated column or Column Footing
3.
Combined Footing
4.
Cantilever (Strap) Footing
5.
Mat (Raft) Foundation
Wall Footing
This type of foundation runs continuous along the direction
of the wall and helps to transmit the load of the wall into the ground. Wall
footing are suitable where loads to be transmitted are small and are economical
in dense sands and gravels. In this type of foundation the width is 2-3 times
the width of the wall at ground level. Wall footing may be constructed through
stone, brick, plain or reinforced cement concrete.
Column Footing
Column footing are suitable and economical for the depth
greater than 1.5m. In this type of foundation the base of the column is
enlarged. Column footing is in the form of flat slab and may be constructed
through plain or reinforced concrete.
Combined Footing
Combined footings are those foundations that are made common
for two or more columns in a row. It is used when the footing for a column may
extend beyond the property line. It is also suitable when the two columns are
closely spaced and the soil on which the structure resist is of low bearing
capacity. It may be rectangular or trapezoidal in shape.
Strap Footing
When an edge footing cannot be extended beyond the property
line the edge footing is linked up with the other interior footing by means of
a strap beam. Such footings are called as strap footing. It is also know as
cantilever footing.
Mat Foundation
A mat foundation is a combined footing which covers the
entire area beneath of a structure and supports all the walls and columns. It
is also known as raft foundation. Mat foundation is applicable when:
Allowable bearing pressure is low.
The structure is heavy.
The site is with highly compressible layer.
The mat foundation can be further classified into following
types:
·
Flat slab type.
·
Flat Slab thickened
under column.
·
Two way beam and
slab type.
·
Flat slab with
pedestals.
·
Rigid frame mat.
·
Piled mat.
Deep Foundation
Deep Foundation are those foundations in which the depth of
the foundation is greater than its width (D>B). The D/B ratio is usually 4-5
for deep foundation. Unlike shallow foundation, the deep foundation transmits
the load of the superstructure vertically to the rock strata lying deep. Deep
foundations are used when the shallow foundation cannot support the load of the
structure.
Classification of Deep
Foundation
The mat foundation can be further classified into following
types:
·
Pile Foundation
·
Pier Foundation
·
Well (Caissons) Foundation
·
Pile Foundation
Pile is a slender member with small area of cross-section
relative to its length. They can transfer load either by friction or by
bearing. Pile foundation are used when:
The load is to be transferred to stronger or less
compressible stratum, preferably rock.
The granular soils need to be compacted.
The horizontal and the inclined forces need to be carried
from the bridge abutments and the retaining walls.
Classification of Pile Foundation
The pile foundation can be further classified into following
types on various basis such as function, material, method of installation which
are listed below:
Based on Function:
·
Bearing piles
·
Friction piles
·
Combined piles (Both bearing and friction)
Based on Material:
·
Timber piles
·
Concrete piles
·
Steel piles
Based on Method of Installation:
·
Large displacement piles
·
Small displacement piles
·
Non-displacement piles
Pier Foundation
Pier foundation are underground cylindrical structural member
that support heavier load of the structure which shallow foundations cannot
resist. Unlike pile foundation, pier foundation can only transfer load by
bearing. Pier foundation are shallower in depth than the pile foundation. Pier
foundation are used when:
The top strata is a decomposed rock underlying as sound rock
strata.
The soil is a stiff clay that occurs large resistance for
driving the bearing pile.
Well (Caissons) Foundation
The term caisson refers to box or a case. These are hollow
inside and are usually constructed at the site and sunk in place into a hard
bearing strata. As they are expensive in construction, they are usually
restricted to major foundation works. Well foundation are suitable when the
soil contains large boulders obstructing the penetration during installation of
pier or pile foundations. Caissons are used for bridge piers, abutments in
rivers and lakes and other shore protection works. They are used to resist
heavy vertical and horizontal loads and are used in the construction of large
water front structures as pump houses.
Classification of Well Foundation
·
Open Caissons
·
Pneumatic Caissons
·
Box Caissons
·
Factors affecting the selection of Foundation:
·
On the basis of ground/soil condition
Shallow foundations are preferred where soil close to the
surface is capable of supporting structure loads.
Where the ground close to the surface is not capable of
supporting structural loads, hard strata is searched for and deep foundation is
required.
Uniform stable ground requires relatively shallow foundation
whereas filled up ground has low bearing capacity thus requires deep
foundation.
On the basis of Loads from Building:
In the case of low-rise building in a larger area, the
extent of loading is relatively low, so shallow foundation can resist the load
from the structures.
In the case of the high-rise building built within less area
have high loads. Therefore, the deep foundation is required as shallow
foundation may not be able to resist such loads of greater intensity.
